This invention relates to variable gain amplifiers and, more particularly, to an improved electronically controlled variable gain amplifier.
Over the years, a number of electronically controlled amplifier designs (hereinafter referred to as ECAs) have been conceived. Generally, these designs may be divided into two basic categories. The first category includes voltage controlled amplifiers (hereinafter referred to as VCAs) in which the gain is a function of an analog control signal.
While there are many textbook designs for VCAs, for the most part they all share several drawbacks, among which are the following. The absolute gain of a VCA is not very accurate, and hence a gain calibration is required in many applications. A further disadvantage is that a percentage of the control signal feeds through to the VCA signal output terminal. This feedthrough represents an error signal which must be compensated for in many application. Yet another disadvantage of VCAs is that in applications that require digital gain control (i.e. microprocessors), a D/A converter is required to convert the gain control word into an analog control signal.
A second category includes digitally controlled amplifiers (DCAs) which also possess several disadvantages. For example, one of the most popular DCA designs employs a multiplying CMOS D/A converter for linear gain control. However, if fine-step logarithmic gain control is required, a high resolution D/A converter and a log lookup table must be added, which substantially increases cost and complexity.
Another popular DCA design employs pulsewidth modulated amplifiers. A major drawback to this type of design is the limited gain resolution, resulting in unsuitability in applications requiring fine-step logarithmic gain control over a wide dynamic range.
Accordingly, it is an object of the present invention to provide a new and improved electronically controlled variable gain amplifier.
It is another object of the invention to provide an electronically controlled variable gain amplifier in which gain is digitally controlled in a logarithmic and intrinsically monotonic fashion.
It is yet another object of the invention to provide an electronically controlled variable gain amplifier in which the absolute gain accuracy and scale factor are controlled only by component tolerances, and in which there is no feedthrough of the control signal to the output signal.